Electrodeposition has become an important method for the application of coatings over the last two decades and continues to grow in popularity because of its efficiency, uniformity and environmental acceptance. Cathodic electrodeposition has become dominant in areas where highly corrosion-resistant coatings are required, such as in primers for automobile bodies and parts. Epoxy based systems provide the best overall performance in this application and are widely used.
Cathodic electrodeposition resins based on conventional epoxies obtained by reacting liquid diglycidyl ethers of bisphenol A with bisphenol A to produce higher molecular weight epoxy resins have known disadvantages. Such products tend to have excessively high softening points resulting in poor flow out. In addition, such products require excessive amounts of solvent during their preparation. In order to improve flow, it has been proposed to modify such conventional epoxy resins by reaction with a diol in the presence of a tertiary amine catalyst. Thus, Bosso et al., U.S. Pat. No. 3,839,252, describes modification with polypropylene glycol. Marchetti et al., U.S. Pat. No. 3,947,339, teaches modification with polyester diols or polytetramethylene glycols. Wismer et al., U.S. Pat. No. 4,419,467, describes still another modification with diols derived from cyclic polyols reacted with ethylene oxide. These various modifications, however, also have disadvantages. Tertiary amines or strong bases are required to effect the reaction between the primary alcohols and the epoxy groups involved. Since these reactions require long cook times, they are subject to gellation because of competitive polymerization of the epoxy groups by the base catalyst. In addition epoxy resins containing low levels of chlorine are required to prevent deactivation of this catalyst.
U.S. Pat. Nos. 4,419,467 and 4,575,523 describe the reaction of an epoxy resin with oxyalkylated diols to form resins useful in electrodeposition. Such reactions have several attendant disadvantages, such as described in U.S. Pat. No. 4,260,720, col. 1, lines 25-51. Use of the glycidyl ethers of such a diol, as described herein, eliminates or greatly reduces these problems.
U.S. Pat. No. 4,260,720 teaches the use of glycidyl ethers of cyclic polyols, including oxyalkylated polyphenols, in combination with polymercapto compounds to form electrodeposition resins. These glycidyl ethers were not used in combination with glycidyl ethers of polyphenols and polyphenols, as described herein, nor were there advantageous properties as modifiers for bisphenol A-based epoxy resins in electrodeposition anticipated, such as improvement in film thickness and appearance.
U.S. Pat. No. 4,698,141 issued Oct. 6, 1987 to Anderson and Hickner discloses an improvement in a method for preparing an advanced epoxy cationic resin from an epoxy-based resin containing oxirane groups by converting at least some of the oxirane groups to cationic groups. The improvement is stated to reside in using as the epoxy-based resin an advanced epoxy resin obtained by reacting in the presence of a suitable catalyst (1) a diglycidyl ether of a polyether polyol such as the condensation product of dipropylene glycol and epichlorohydrin having an epoxy equivalent weight of 185, (2) a diglycidyl ether of a dihydric phenol such as a diglycidyl ether of bisphenol A and (3) a dihydric phenol such as bisphenol A and optionally a capping agent such as p-nonylphenol.
U.S. Pat. No. 4,868,230 issued Sep. 19, 1989 to Rao and Hickner discloses an improvement in a method for preparing an advanced epoxy cationic resin from an epoxy-based resin containing oxirane groups by converting at least some of the oxirane groups to cationic groups. The improvement is stated to reside in using as the epoxy-based resin an advanced epoxy resin obtained by reacting in the presence of a suitable catalyst (1) a diglycidyl ether of an aliphatic diol which is essentially free of ether oxygen atoms, such as a diglycidyl ether of 1,4-butanediol, (2) a diglycidyl ether of a dihydric phenol such as a diglycidyl ether of bisphenol A and (3) a dihydric phenol such as bisphenol A and optionally a capping agent such as p-nonylphenol.
Many coating formulations applied by electrodeposition include pigments to provide color, or opacity or application or film properties. U.S. Pat. No. 3,936,405, Sturni et al., describes pigment grinding vehicles especially useful in preparing stable, aqueous pigment dispersions for water-dispersible coating systems, particularly for application by electrodeposition. The final electrodepositable compositions, as described, contain the pigment dispersion and an ammonium or amine salt group solubilized cationic electrodepositable epoxy-containing vehicle resin and other ingredients typically used in electrodepositable compositions. Among the kinds of resins used are various polyepoxides such as polyglycidyl ethers of polyphenols, polyglycidyl ethers of polyhydric alcohols and polyepoxides having oxyalkylene groups in the opoxy molecule.
The automobile industry still has needs in the areas of controlled film thickness and lower temperature cure systems. The ability to build thicker, uniform films which are smooth and free of defects will allow the elimination of an intermediate layer of paint known as a primer surface or spray primer, previously required to yield a sufficiently smooth surface for the topcoat. Such an elimination results in removal of one paint cycle and provides more efficient operations.
European Patent Application 0,253,405 published Jan. 20, 1988 discloses cationic resins prepared from an advanced epoxy resin prepared in one step by reacting in the presence of a suitable catalyst, (A) a composition comprising (1) at least one diglycidyl ether of a polyol and (2) at least one diglycidyl ether of a dihydric phenol with (B) at least one dihydric phenol and (C) optionally, a monofunctional capping agent.
The cationic advanced epoxy resins prepared by the procedure of the European Patent Application 0,253,405 satisfies many of the requirements desired by the automobile industry. Nevertheless, it would be desired to have available cationic resins which provide further improvements in coating properties such as higher rupture voltage, lower bath conductivity, and the attendant improvement in appearance at higher coating voltages, and the like. Thicker electrocoat primers may also provide improved corrosion resistance.